Adjustable inductance



Apri] ll, 1939.

P. S. DARNELL ADJUSTABLE INDUCTANCE Filed May l5, 1937 o o o 4.4249

/NVENTOR BVRSDRNELL A TTORNE Y Patented Apr. i1, 1939 i 2,154,232

UNITED- STATES PATENT ori-Ica ADJUSTABLE INDUUIANCE Paul S. Darnell,South Orange, N. J., signor to Bell Telephone Laboratories,Incorporated, New York, N.- Y., a corporation of New York Application my1s, rim, sei-isi No. 14am 4 emma (ci. 1v1-11s) This invention relates toadjustable inductances type disclosed in the W. C. Ellis Patent1,943,115, and more particularly to toroidal coils, the inissued January9, 1934. ductance of which may be varied. The core as shown more clearlyin -Il'igs. 2 an Toroidal coils have gone into wide use as load- 3, hasa tapered hole 6 drilled completely through 5 ing or lter coils,transformers and the like due the core, the axis of the hole coincidingwith a 5 to the inherent eiliciency of the magnetic circuit radius ofthe torus.l Within the hole is located and other recognized advantages.However, the a movable disc 1 of magnetic material mounted use of suchcoils in places where a high degree of on the end of a screw l ofnon-magnetic mateaccuracy of inductance is required has been limrialsuch as brass, which screw is adapted to move 1o ited due to thedliilculty of adjusting the inductin an internally threaded4 brasssleeve 9 suitably l0 ance of such coils to the accuracy required. Evensecured to a plate I of Bakelite or other insuwith care in manufactureit has been found that latlng material which serves as a closure for thecoils will diier suillciently in inductance from the outer end of hole 8while the inner end of the required value to maketheir use dinlcult forcerhole is closed by a plate Il of insulating material l tain purposes.For example, placing toroidal such as ilbre. Plates I0 and Il may besuitably l5 coils in an enclosed case, impregnatlng the windheld inplace. for example, by being cemented to ings or mounting the coils inproximity to other the surfaces around the core opening 0. It willapparatus may vary their inductance. be apparent that by turning screw 8the disc 1 An object of this invention is a toroidal coll of magneticmaterial may be moved upwardly or 'whose inductance may be varied.downwardly to the dotted positions I2, I 3, its up- 20 Another object ofthe invention is a toroidal per position being reached when the upperedge coil whose inductance may be accurately adjusted of disc 1 strikesplate I0, and the lower position after the wound core has beenimpregnated with being reached when the lower surface of disc 1 asuitable compound. strikes cover plate Il. It thus will be noted that Inaccordance with the preferred embodiment the movement of disc 1 isconflned to a region ly- 25 of this invention a toroidal corevofmagnetic maing Wholly Within the periphery 0f the cere. 'I'hc terial isprovided with a cylindrical or tapered dimensions of disc 1 arepreferably such that the hole whose axis coincides substantially with aside walls of the disc fit snugly against the walls radius of the torusand within the hole is adof hole 6 when the disc is in its lowermostD081- justably mounted a relatively thin disc or plate tion Il. 30 ofmagnetic material with the axis of the disc Since as previeilsly statedthe disc 1 moves ensubstantially coinciding with the axis of the hole.tirely within the limits of the core surface, it A winding of a suitablenumber of turns is then follows that the multi-layer winding 4 may beapplied to the core and the wound core lmpreguniformly distributedaround the toroidal core nated in the usual manner and suitably mountedexcept for the Space Occupied by Sleeve 9, so that 35 in the casingafter which the inductance of the the winding Space 10st by the Se 0fthe adjustwinding may be accurately adjusted to the reable feature ofthis invention is not determined quired value by the movement of thedisc. The by the diameter of hole 6 but merely by the diammaxlmuminductance will occur when the disc is eter of the small sleeve 9.

at the inner periphery of the torus while the min- After the hole 6 hasbeen drilled in the core, 40 imum inductance will occur when the disc isat the core may be Coated With a Suitable enamel to the outer periphery,intermediate positions of the prevent wax during the impregnatingprocess disc giving intermediate values of inductance. from beingabsorbed into the dust core and alter- Referring to the drawing, ing itspermeability-temperature characteristic.

Fig. 1 is a perspective view in partial section and also to prevent thewax from entering the 45 of a toroidal core'inductance coil embodyingfeahole t. After the enameling the disc assembly tures of thisinvention; 1, Il, 9, III may then be mounted in place after Fig. 2 is alongitudinal sectional view of a por which the core is. ready to bewound with a suittion of the wound core of Fig. l; and able number ofturns and layers to give the ap- Fig. 3 is a cross-sectional view of thecore of proximate desired inductance with thedisc 1in its 60 Fig. ltaken along the line I-S of Fig. 2. middle position. The wound core isthen mount- Referring now to Fig. ll a multi-layer winding ed within asuitable casing I! and the remaining l is shown uniformly mounted upon atoroidal space fllled with a suitable potting compound such core I madepreferably of highly compressed inas wax, or the coil may be centered inthe case sulated magnetic particles, for example, of the in any othersuitable manner. The ends of the winding are brought out to terminals iland l1 mounted at any convenient place on a suitable terminal platewhile the head Il of the adjusting screw l projects through an aperturein the case to permit adjustment of disc 'i after the wound core hasbeen potted. By means of a screwdriver or other tool the inductance maybe brought accurately to the desired value by turning screw! in theproper direction alter which the screw l may be locked in any suitablemanner to prevent further turning.

It will be noted that when the disc is in its lowermost position thereis practically no alrgap between the disc and the main core since inthis position the disc ilts snugly against the walls of hole l, but asthe disc is moved upwardly there is a gradually increasing air-gapbetween the disc and the core due to the increasing size of the hole.

In explanation of the change in inductance with change in position ofdisc i we may assume that hole I is cylindrical and that disc 'l is alsocylindrical and fits snugly into the hole in all positions. It can beshown under such conditions that the ilux density at the inner peripheryof the torus is substantially times that at the outer periphery, where nis the inside radius and n the outside radius of the torus. Hence, bycompleting the magnetic circuit at the inner periphery the ilux in thecore will be greater than if the magnetic circuit is completed at theouter periphery. When both the hole and the disc are slightly tapered asshown in the drawing with the disc fitting snugly into the hole only atthe inner periphery of the torus. the inductance variations result tromtwo actions, the variation in air-gap and the alteration in iiuxdensity, and the two actions are mutually aiding, thereby increasing theoverall variation of the inductance with change in position oi' disc l.

As previously stated, the disc 1 is preferably of higher permeabilitythan the main core although both the core and the disc i may be composedof iinely divided magnetic material insulated and compressed inaccordance with the above-mentioned Ellis patent, where the magneticmaterial employed may be molybdeninn permalloy as disclosed in G. W.Elmen, U. S. Patent 1,768,443 issued June 24, 1930. In one example, themagnetic material of the core 5 had an effective permeability of 13.5while the magnetic material of disc 1 had an eHective permeability of125. With the construction shown in the drawing, it hm been found that avariation of 2 per cent of the inductance of the coil can be readilysecured by the described arrangement and it has also been found thatwith the shown construction the percentage change in inductance ispractically linear with change in position of disc i along the axis oftapered hole I. The thickness of disc 1 should preferably be smallcompared to its length of travel; for example, it has been foundsatisfactory to have the thickness of the disc about one-sixth of itslength of travel.

While it is preferred to have hole l slightly tapered it is alsocontemplated that both the hole l and disc 1 may be cylindrical.

Itistobeunderstoodthatthisinventionis applicable to-a coil having aplurality o! windingsandisnotlimitedtoacollhaving onlya single windingas described. lhe adjustable feature may also be modiiied from thatshown and the invention is, therefore, to be limited only by the scopeof the appended claims.

What is claimed is:

1. An inductance device comprising a toroidalY core of magneticmaterial, a winding on said core, said core having a hollow chamberextending substantially between the inner and outer peripheries of saidcore, a movable member of magnetic material located wholly within saidchamber, means for moving said member along an axis extending betweenthe inner and outer peripheries of said core, said member having in thedirection oi its movement a thickness substantially less than thedistance between the inner periphery and the outer periphery oi saidcore, and means for coniining said movement between the inner peripheryand the .outer periphery of said core.

2. An inductance device comprising a toroidal core of magnetic material,a winding on said core, said core having a hole extending substantiallybetween the inner and outer peripheries of said toroid, with the axis ofsaid hole substantially coinciding with a radius of said toroid, a thinplate of magnetic material mounted within said hole with the platelocated substantially at right angles to the axis of said hole, anon-magnetic cover for the inner and outer ends of said hole, saidwinding extending over a substantial proportion of said cover, and meansprojecting through said cover for moving said plate in a direction alongthe axis of said hole.

3. An adjustable inductance in accordance withclaim2lnwhichsaidplateisadisc andinwhich the surface defining said holeconstitutes a portion of the surface of a cone with its greatestdiameter at the outer periphery of the toroid, the diameter or said discbeing substantially equal to the diameter of said hole near the innerperiphery of said toroid.

4. An adjustable inductance device comprising a toroidal core, a windingon said core, said core having a hole extending through said toroid withthe axis of the hole substantially coinciding with the radius of thetoroid, a nonmagnetic cover for the inner end oi' said hole, anon-magnetic cover for the outer end of said hole, said outer coverhaving an aperture substantially coinciding with the axis of said hole.a sleeve extending beyond the outer periphery of said toroid with theaxis of said sleeve substantially coinciding with the axis of said hole,a screw extending through said sleeve into said hole and a disc ofmagnetic material within said hole and mounted on said screw.

PAUL S. DARNHL.

